Along with recent dissemination of networks, documents having heretofore distributed in form of printed matters have come to be distributed in form of so-called electronic documents. Additionally, more and more books and magazines are also becoming delivered in form of so-called electronic publications.
Conventional way of accessing to these kinds of information is to read from CRT or liquid crystal displays of computers. However, it is pointed out that emission-type displays cause much fatigue due to ergonomic reasons, and users cannot withstand long-time reading. Additionally, there is the disadvantage that a user can read it only at the place where a computer is set.
Together with recent distribution of note-type computers, there are devices usable as portable displays. However, also these devices cannot be used for reading over several hours or more because of the problem of power consumption in addition to the reason that the display is of an emission type. Reflection-type liquid crystal displays have also been developed recently, and it will be possible to drive them with low power. However, reflectance of liquid crystal under no display (black-and-white display) is 30%, and visibility of such displays is much worse than prints on paper. Therefore, users are liable to fatigue and cannot withstand long-time reading.
To deal with these problems, devices called paper-like displays or electronic paper are under development. They color their representations mainly by moving color particles between electrodes by electrophoresis or by rotating dichromatic particles in an electric field. These methods, however, involve the problems that gaps among particles absorb light and thereby degrade the contrast, and a writing speed acceptable for practical use (within one second) cannot be attained unless raising the drive voltage to 100 V or more.
Electrochromic display apparatuses (ECD) generating color by electrochemical operations are superior to the electrophoretic schemes from the viewpoint of high contrast, and have already been used in practical light control glass and watch or clock displays. However, since light control glass and clock or watch displays do not originally need the matrix drive, they are not applicable to the use of display such as electronic paper. Additionally, quality level of black is bad, in general, and their reflectance is still low.
Displays such as electronic paper are inevitably exposed continuously to light such as sunlight or room light because of their purposes of use. In electrochromic display apparatuses of the type practically used as light control glass and clock displays, certain organic materials are used for forming black portions. Generally, however, organic materials exhibits poor light resistance, and are bronzed and degraded in black optical density after long use. Additionally, a matrix-driven display apparatus taught by Japanese Patent Publication No. hei 4-73764 is also known. However, the drive device merely composes a part of the liquid crystal display apparatus.
In view of these technical problems, it is an object of the invention to provide an electrochromic display device and an electrochromic display apparatus operative by matrix driving and capable of enhancing the contrast and the black optical density.
A further object of the invention is to provide an electrochromic display device and an electrochromic display apparatus capable of maintaining the black optical density high without the problem of bronzing even after long-time use.